Epoxy resin have been widely employed in the fields of adhesives for paint, sealing agents, structural materials, etc. due to their excellent heat resistance, electrically insulating property, chemical resistance and mechanical characteristics. Particularly, composite materials reinforced with carbon fibers (CFRP) have been recently employed as structural materials for space- and air-crafts, base materials for transportation industries such as railways and automobiles, etc., materials for leisures such as golf shafts, rods, ski boards, etc. because CRFP possess mechanical strength and modulus of elasticity which are equivalent to or stronger than those of metals and can render the weight light. Thus, great developments are further expected.
At present, polyepoxy compounds which are employd as matrix resins for CFRP include diglycidyl ether of bisphenol A (Epicoat 828, Epicoat 1004, etc.; Yuka Shell Epoxy Co., Ltd., trademarks), polyepoxide of aminophenol (ELF-120; Sumitomo Chemical Co., Ltd., trademark), tetraepoxide of methylenedianiline (YH-434, Toto Kasei Co,. Ltd., trademark), phenol novolac polyepoxide (Epicoat 154; Yuka Shell Epoxy Co., Ltd., trademark), ortho-cresol novolac epoxide (EOCN 104 S: Nippon kayaku Co., Ltd., trademark), etc.
However, CFRP in which these polyepoxy compounds are used as matrix resins are not sufficiently satisfied with performances such as elongation (flexibility), heat resistance, modulus of elasticity, low water absorbability etc., which are required in large scale elements such as primary structural materials, etc. for aircrafts.
A polyepoxy compound having a spiroacetal ring is known as an polyepoxy compound which gives hardened product having good flexibility.
For example, U.S. Pat. No. 3,128,225 discloses a polyepoxy compound represented by the formula: ##STR2##
However, the thermal distortion temperature of the hardened product obtained therefrom is in the range of 147.degree. to 170.degree. C., which is poor in heat resistance as matrix resins for use of primary structural materials for aircrafts.
Further, U.S. Pat. Nos. 3,347,871 and 3,388,098 disclose a polyepoxy compound represented by the formula: ##STR3## wherein Y is H, Cl or CH.sub.3 and n is 0 or an integer of 1 or 2, which is produced by (A) reacting a monovalent phenol having an aldehyde group at the p-position to the phenolic hydroxy group with pentaerythritol and then (B) reacting epichlorohydrin with the resulting divalent phenol. While this polyepoxy compound gives a hardened product having excellent heat resistance and impact resistance, it involves disadvantages that not only the compound is required to further improve the flexibility and the solubility in widely used solvents, e.g., acetone, methyl ethyl ketone, methyl isobutyl ketone, toluene, ethyl acetate, tetrahydrofuran, etc. is poor but also mixtures with other epoxy resins widely used cause phase separation due to its poor compatibility with these epoxy resins, e.g., diglycidyl ether of bisphenol A, novolac type epoxy resins, etc.
In order to overcome these problems, the present inventors have previously proposed the epoxy compound having the spiroacetal ring represented by the formula (I): ##STR4## in Japanese Patent Application No. 21970/82.
The spiroacetal ring-containing epoxy compound has excellent solubility in solvents and gives a hardened product having excellent flexibility but is further required to improve a water absorption rate.
A high water absorption rate of the hardened product results in reduction of strength.